The invention relates to a process for the production of cushions of plastic foam for seats. The cushions may be used to form either a seat portion on a backrest portion of a seat, but for convenience and conciseness in this specification the term seat cushion will be used broadly to designate those potential situations of use.
In order for seat cushions to enjoy a higher degree of stability and strength in their side regions, for example outer cheek portions, seat cushions may be produced from plastic foam in such a way that the side regions thereof are made using a plastic foam which has a higher level of compression hardness than the middle region of the seat cushion which is thus softer.
In a known process for producing seat cushions from plastic foam of that nature, that is to say, having a softer middle region and harder side regions to provide additional support at the sides of the seat cushion, two mixing heads are employed for forming the plastic foam mixture and injecting it into a mold. Thus, the components of the plastic foam which may be for example a polyurethane foam are mixed in the two mixing heads, and caused to foam in the mold which corresponds to the configuration of the seat cushion to be produced. That process provides that the two mixing heads are firstly positioned above the middle of the mold which is in an open condition. The above-mentioned components of the plastic foam are then mixed in the mixing heads in such a way that a plastic foam with a low degree of compression hardness can be caused to foam and be molded in the mold. After that plastic foam has been injected into the mold, the two mixing heads are moved away from each other in mutually opposite directions until the two mixing heads are positioned over the lateral regions of the open mold. With the two mixing heads in that position, the components making up the plastic foam are altered in such a way as to produce a plastic foam with a higher degree of compression hardness, which can thus be caused to foam up in the mold.
The above-indicated steps in that process, namely foaming a plastic foam with a low degree of compression hardness in the middle region of the open mold, moving the two mixing heads away from each other from the middle region of the mold to the laterally mutually oppositely disposed regions of the mold and then molding a plastic foam with a higher degree of compression hardness in the lateral regions of the mold, take place at very short intervals of time so that the plastic foams, with their different degrees of hardness, react simultaneously and the increase in volume of the two plastic foams also occurs at the same time.
It should be noted in connection with that process that ribs which are provided in the open mold between the middle region and the lateral regions thereof ensure that the plastic foams, with their differing degrees of compression hardness, do not mix with each other in a liquid condition directly after having been introduced into the open mold in order to foam up therein.
It will be clear from the foregoing discussion that that known process requires two mixing heads while in addition it is only possible for the plastic material to be introduced into a mold which is in an open condition, for the purposes of foaming therein, because the mixing heads move over the mold. However, the step of introducing plastic material into an open mold for the purposes of forming foam therein may give rise to problems in particular when using shallow molds to produce molded articles which are not of substantial depth, because of the possibility of the plastic foam overflowing out of the mold and thus fouling the outside of the mold and possibly the area around it.
Another process for the production of seat cushions from plastic foam requires only a single mixing head. In that process, the mixing head is moved continuously over the mold which is open, while at the same time as the mixing head moves, the compression hardness of the plastic foam for making up the seat cushion is adjusted and controlled, by altering the ratio between the components used for making up the plastic foam, in accordance with the respective position of the mixing head in relation to the mold. However, continuously adjusting the ratio between the components in the mixture for making the plastic foam in dependence on the position of a continuously moving mixing head involves a complex and therefore very expensive construction. Likewise this process is also such that the plastic foam can only be introduced into and foamed in an open mold because the mixing head is still required to move over the mold in order to produce the different hardnesses of the middle and side regions of the seat cushion molded in the mold. For that reason also the process just discussed is very severely limited in regard to its suitability for producing moldings of shallow depth.